Fuel cell stack flow diversion

ABSTRACT

A fuel cell system has a compressor delivering compressed gas to a fuel cell stack and a control valve affecting the flow of compressed gas. A load dump condition is determined for the fuel cell stack. The flow through the compressor is increased and the additional flow diverted away from the fuel cell stack by the control valve to provide additional load for the fuel cell stack. The fuel cell stack may then be operated at a higher output power for the purpose of generating more waste heat to more rapidly warm itself.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/839,838,filed Aug. 16, 2007, the entire contents of which are herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support under Contract No.DE-FC36-04G014287. The Government has certain rights to the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the use of compressors and compressed gas infuel cell systems.

2. Background Art

Fuel cell systems produce electrical energy by combining fuel and anoxidant in a fuel cell stack. In one form of fuel cell system, the fuelis hydrogen and the oxidant is oxygen, which may be mixed with othergases as in air. The oxidant is typically gaseous and is often deliveredto the fuel cell stack as a compressed flow.

Typically, fuel cell stacks operate more efficiently under certainoperating conditions, including fuel cell stack temperature. Inparticular, it is desirable for the fuel cell stack to operate at orabove a particular temperature, which may be above ambient temperature.Therefore, there is a need to heat the fuel cell stack at various timessuch as, for example, during startup.

Different types of compressors may be used to provide oxidant to thefuel cell stack. For example, non-positive displacement compressors aresometimes used for a variety of reasons such as size, weight,efficiency, noise, vibration, and harshness characteristics. However,non-positive displacement compressors may operate in an undesirablecondition known as surge. Surge occurs when the compressor is operatedat low flow rates in combination with a high ratio of output pressure toinput pressure. Under these conditions, surge may result in vibrationswhich can lead to poor operation, malfunction system damage, and thelike. In fuel cell systems that use ambient air to provide the oxidant,variations in air density and pressure can affect compressorperformance. This is particularly true at higher elevations, where theonset of surge is more likely.

Fuel cell stacks are typically placed in a housing. Unwanted gasses mayaccumulate in the housing, requiring some mechanisms to vent or purgethe unwanted gasses.

Fuel cell systems are often part of a larger system such as, forexample, an automotive vehicle. These larger systems often requirevarious environmental modification systems that could benefit fromsynergistic operation with the fuel cell system.

Accordingly, a need exists for improved fuel cell system operation whichaddresses some or all of the above issues without unduly affecting cost,complexity, performance, and the like.

SUMMARY OF THE INVENTION

The present invention provides a control valve to affect the flow ofcompressed gas in a fuel cell system.

In one embodiment, a compressor supplies compressed gas to the fuel cellstack. The compressor may be used as a load dump for energy produced bythe fuel cell stack. In this case, the compressor generates an excessflow of compressed gas which is diverted by a control valve away fromthe fuel cell stack. In one application, excess work done by the fuelcell stack to power the compressor generates heat which warms the fuelcell stack.

Control logic may be used to manage the compressor and the control valveso as to maintain efficient fuel cell stack operating conditions. Thiscontrol logic may receive as input one or more conditions of thecompressed gas, ambient air, compressor, fuel cell stack, control valve,and the like.

The control valve may be used to avoid a surge condition in thecompressor. The compressor may generate an increased flow to avoidsurge. The control valve may then divert the increased flow away fromthe fuel cell stack.

According to an embodiment of the present invention, componentryincorporated for expending electrical energy from the fuel cell stackmay be reduced or eliminated by running the compressor at a level abovethat needed to supply compressed gas to the fuel cell stack anddiverting excess flow away from the fuel cell stack.

Another embodiment involves utilizing compressed gas diverted away fromthe fuel cell stack. This diverted gas may be used to modify theenvironmental conditions of a wide variety of systems such as, forexample, a passenger compartment, a radiator, and the like. The divertedgas may also be used to evacuate gases from the fuel cell stack housingor enclosure.

In another embodiment, the fuel cell system may be utilized in anautomotive vehicle.

Other aspects, features, and uses of the disclosed inventions willbecome apparent to one skilled in the art from a study of the followingdescription and associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a fuel cell system accordingto an embodiment of the present invention;

FIG. 2 is a flow diagram illustrating operation of a fuel cell systemaccording to an embodiment of the present invention;

FIG. 3 is a graph illustrating surge avoidance according to anembodiment of the present invention; and

FIG. 4 is a schematic diagram illustrating an automotive vehicleaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for the claims and/or as a representative basis forteaching one skilled in the art to utilize the present invention.

Referring now to FIG. 1, a schematic diagram illustrating a fuel cellsystem according to an embodiment of the present invention is shown. Asimplified oxidant path of a fuel cell system, shown generally by 20,includes fuel cell stack 22. Fuel cell stack 22 generates electricalenergy, shown as 24, by combining ionized fuel and oxidant. The oxidantis provided by fuel cell stack inlet 26. Unused oxidant, and other gasesin some systems, exits the fuel cell stack at outlet 28. A wide varietyof fuel cell types are available based on different fuels and oxidants,cell stack configuration, desired power output, fuel cell application,and the like. As will be recognized by one of ordinary skill in the art,the present invention does not depend upon the type or construction ofthe fuel cell stack used.

Fuel cell system 20 also includes compressor 30 and control valve 32.Compressor 30 receives gas at compressor inlet 34 and producescompressed gas at compressor outlet 36. Compressor 30 runs on electricalenergy, shown by 38, which is at least a portion of fuel cell stackgenerated electrical energy 24. The operation of compressor 30 iscontrolled by compressor control signal 40 provided by control logic 42.Compressor control signal 40 may be one or more of analog voltagesignals, analog current signals, pulse width modulated signals, digitalsignals, and the like. Compressor electrical supply may be AC or DC andmay be controlled by control logic 40 so as to provide compressorcontrol signal 40. A wide variety of compressors are available for usein the present invention depending upon the type of oxidant used; flowparameters including pressure, temperature, and velocity; amount ofcompression needed; fuel cell system application; and the like. In oneembodiment, compressor 30 is a centrifugal compressor. As will berecognized by one of ordinary skill in the art, a wide variety ofcompressor types and configurations may be used in the presentinvention.

Control valve 32 is disposed in the flow path between compressor outlet36 and fuel cell stack inlet 26. Control valve 32 diverts flow fromcompressor 30 away from fuel cell stack 22. Control valve 32 iscontrolled by valve control signal 44 from control logic 42. Controlsignal 44 may be any type of analog or digital signal depending upon thetype of control valve 32 chosen, including electrical, magnetic,pneumatic, hydraulic, optical, and the like. In addition, any suitabletype of control valve 32 may be used. While a single control valve isillustrated, the term control valve includes one or more control valvescontrolled by one or more valve control signals. As will be recognizedby one of ordinary skill in the art, a wide variety of control valvetypes and configurations may be used to implement the present invention.Moreover, various other components may be disposed in the flow path,including intercooler, filters, water injectors, humidifiers, and thelike.

Control logic 42 generates compressor control signal 38 and valvecontrol signal 44 based on one or more control inputs 46, the specificconnections of which are not shown for clarity. Control inputs mayinclude flow parameters including mass flow rate, volume flow rate,velocity, temperature, and the like, at various locations in fuel cellsystem 20 such as compressor inlet 34, compressor outlet 36, fuel cellstack inlet 26, fuel cell stack outlet 26, ambient, and the like.Control logic 42 may also monitor various components in fuel cell system20 including fuel cell stack 22, compressor 30, control valve 32, andthe like. Control inputs 46 may include inputs from a user or anothercontroller. In one embodiment, control logic 46 measures a temperaturerelated to the operation of fuel cell stack 22 and uses compressor 30 asan electrical load for warming fuel cell stack 22. Control logic 42 maybe implemented as a computer executing software, as programmable logic,as discrete logic components, as electromechanical, hydraulic, orpneumatic systems, any combination of these, and the like. Control logic42 may be a single unit or may be distributed between or amongst variousunits. As will be recognized by one of ordinary skill in the art, thepresent invention may be implemented in a wide variety of control logictypes and configurations.

Referring now to FIG. 2, a flow diagram illustrating operation of a fuelcell system according to an embodiment of the present invention isshown. As will be appreciated by one of ordinary skill in the art, theoperations illustrated are not necessarily sequential operations. Theorder of steps may be modified within the spirit and scope of thepresent invention and the order shown here is for logical presentation.Also, methods illustrated may be implemented by any combination ofhardware, software, firmware, and the like, at one location ordistributed. The present invention transcends any particularimplementation and the embodiments are shown in sequential flow chartform for ease of illustration.

The fuel cell stack generates electricity to drive the compressor, as inblock 60. The compressor provides a flow of compressed gas to the fuelcell stack at a first flow rate, as in block 62. This flow rate may bedetermined by one or more of a variety of techniques, including directlyor indirectly measuring the mass flow rate, the volumetric flow rate,and the like.

A check is made to determine whether or not to increase the flow rateabove the flow needed by the fuel cell stack, as in block 64. Thisincrease may be triggered, for example, by the need to increase the loadon the fuel cell stack. One purpose for increasing the load may be togenerate heat for warming the fuel cell stack. Another purpose forincreasing the load may be to test the fuel cell stack and/or some othercomponent of the fuel cell system. In addition, or rather than,responding to a need to increase fuel cell stack load, the flow rate maybe increased so as to generate excess flow for purposes other than toprovide oxidant to the fuel cell stack. This excess flow may be used tomodify an environmental condition of an element within or outside of thefuel cell system.

If flow is to be increased, the fuel stack electrical output isincreased to drive the compressor, as in block 66. Additional flow isprovided from the compressor, as in block 68. The additional flow isdiverted away from the fuel cell stack, as in block 70. The divertedflow may be directly or indirectly returned to the compressor or ventedto the atmosphere. The diverted flow may also be used for a variety ofpurposes, as disclosed elsewhere herein.

Returning again to FIG. 1, various embodiments for use of diverted flowaccording to the present invention are shown. Fuel cell stack 22 iscontained in housing 80. Operation of fuel cell stack may cause theformation of gases within housing 80, shown generally by 82. Divertedflow 84 from control valve 32 may be routed into housing 80 to purgehousing gases 82 from housing 80.

Diverted flow 84 may also be routed to radiator 86, heat exchanger 88,direct application 90, and the like for modifying one or moreenvironmental parameters. Radiator 86 may use diverted flow 84 to heator cool a liquid such as, for example, coolant used to regulate thetemperature of an internal combustion engine or an electronic circuit.Radiator 86 may also function as a heat sink for electrical componentrycooled by diverted flow 84. Heat exchanger 88 may provide heat fromdiverted flow 84 to a surrounding environment such as, for example, apassenger compartment (not shown for clarity). Direct application 90provides diverted flow 84 directly into an environment to be modified.The path of diverted flow 84 may include various other components suchas diffusers, expanders, intercoolers, humidifiers, and the like forregulating properties of diverted flow 84 prior to use by radiator 86,heat exchanger 88, or direct application 90.

Output flow, shown generally by 92, can include one or more of the flowfrom fuel stack outlet 28 and the diverted flow 84 uses such as purginghousing 80, radiator 86, heat exchanger 88, direct application 90, andthe like. Some or all of output flow 92 may be returned to compressorinlet 34, may be vented to ambient, may be routed for other uses, andthe like.

Referring now to FIG. 3, a graph illustrating surge avoidance accordingto an embodiment of the present invention is shown. A compressoroperating map, shown generally by 100, plots mass flow rate through anexemplary compressor as a function of compressor pressure ratio. Thecompressor pressure ratio refers to the ratio of air pressure exitingthe exemplary compressor outlet to air pressure entering the exemplarycompressor inlet.

Compressor pressure ratio, being a function of mass flow, is alsodependent upon the velocity with which the compressor rotates itsimpellers. Six exemplary compressor operating lines with correspondingangular velocities, indicated on compressor operating map 100, aremarked by respective 30, 45, 60, 75, 90, and 97 thousands of revolutionsper minute (krpm). Compressor operating maps, such as 100, are typicallycreated by setting a compressor at a constant angular velocity andsubsequently varying mass flow through the compressor.

Compressor surge typically occurs when a compressor operates at low flowrates in combination with relatively high compressor pressure ratios.For the example provided, operating the compressor to the left of surgeline 102 will more than likely result in compressor 30 experiencingsurge. This surge condition may cause unsteady aerodynamic loading,observed in flow and pressure oscillation, which may result in damage toequipment or otherwise affect operation.

The present invention may be used to avoid a surge condition in thecompressor. For example, various components in the fuel cell system maybe monitored, such as the compressor inlet and compressor outlet.Preexisting compressor performance data may be stored such as, forexample, that shown on compressor operating map 100. Surge line 102represents the pressure at which surge can be expected to occur for agiven mass flow rate. When the compressor operates in an interventionregion, shown generally by shaded region 104, the compressor may becontrolled to increase output flow, thereby avoiding or removing surge.As previously described, this increased flow may be diverted from thefuel cell stack. For example, if the compressor is operating at surgecondition 106, mass flow can be increased through the compressor. Asflow is increased, operation moves along operating line 108 until stableflow and pressure condition 110 is reached beyond surge line 102.

Control of flow rate may also prevent the compressor from initiallyreaching surge. For this reason, intervention region 104 may encompasssurge line 102 and overlap a portion of conditions in which compressor30 may be operating satisfactorily. For example, if the compressor isoperating at 75 krpm and its mass flow rate drops to 120 kg/h, thecompressor flow may be increased before the compressor ever reaches asurge condition.

Referring now to FIG. 4, a schematic diagram illustrating an automotivevehicle utilizing a fuel cell system according to an embodiment of thepresent invention is shown. An automotive vehicle, shown generally by120, is driven by electric motor 122 receiving fuel cell stack generatedelectrical energy 24. Electric motor 122 may drive axle 124 extendingbetween wheels 126. While electric motor 122 is shown to propel vehiclebody 120, one skilled in the art will realize that there are countlessalternative applications onboard a motor vehicle which involve anelectric motor driving a component. These applications may includeoperating, for example, power windows, an automatic vehicle closure,power steering, or a plow attached to a vehicle body.

Air filter 128 may be included to purify ambient air 130 prior to itsuse in fuel cell system 20. Compressor 30, also driven by fuel cellstack generated electrical energy 24, compresses ambient air 130. Airexiting compressor 30 may have an excessive temperature unsuitable forfurther usage. Air intercooler 132 may be included to modify thetemperature of air flow exiting compressor 30 back within a usablerange.

In the embodiment shown in FIG. 4, fuel cell system 20 includes controlvalves 32 a and 32 b. Control logic 42 operates control valves 32 a and32 b for a variety of purposes, such as to provide fuel cell stack 22with substantially constant absolute pressure compressed air flow forparticular operating conditions.

In one application, fuel cell stack 22 is operated at a higher outputpower solely for the purpose of generating more waste heat to warmitself and any system coolant volume faster. A portion of the outputpower of fuel cell stack is dumped into compressor 30 for operation at ahigher speed. For example, a 50% efficient fuel cell stack generates 1kW of heat for every 1 kW of output power.

Vehicle 120 includes passenger compartment 134. Passenger compartment134 may utilize diverted flow 84 passing through heat exchanger 88 tochange an environmental condition of passenger compartment 134.Alternatively, heat exchanger 88 may use air flow prior to, or insteadof, passing through air intercooler 132.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method of operating a fuel cell stack used for generatingelectrical energy, at least a portion of the generated electrical energypowering a compressor for providing compressed gas to the fuel cellstack at a first flow rate, the method comprising: providing compressedgas flow from the compressor at a second flow rate greater than thefirst flow rate; generating additional electrical energy by the fuelcell stack to power the compressor to provide the compressed gas flow atthe second flow rate; and diverting gas corresponding to a differencebetween the second flow rate and the first flow rate through a controlvalve before the compressed gas reaches the fuel cell stack; therebyusing the compressor as an additional load on the fuel cell stack abovewhat is required to provide the fuel cell stack with compressed gas. 2.The method of claim 1 further comprising warming the fuel cell stackwith heat produced by generating the additional electrical energy. 3.The method of claim 1 further comprising determining when a surgecondition exists in the compressor, increasing flow through thecompressor to avoid compressor surge, and selectively operating thecontrol valve to divert the increased flow away from the fuel cellstack.
 4. The method of claim 1 further comprising changing anenvironmental condition of a passenger compartment with the divertedcompressed gas.
 5. The method of claim 1 further comprising changing thetemperature of a radiator with the diverted compressed gas.
 6. Themethod of claim 1, wherein the compressed gas reaching the fuel cellstack has a substantially constant absolute pressure.
 7. The method ofclaim 1 further comprising monitoring at least one property of thecompressed gas and regulating the control valve based on the monitoredproperty.